When faced with varying demands and situations, the human cognitive system often responds with i remarkable flexibility. Performance is adjusted to match task goals. However, the system is also fallible and there are times when performance deteriorates. In the proposed research, we will investigate flexibility and fallibility using the approach of cognitive psychophysiology. This approach utilizes measures of electrical brain activity and electromyographic activity as well as measures of overt behavior (the speed and accuracy of responses). Because the psychophysiological measures are related to specific aspects of human information processing, they enable the investigator to localize the processes associated with flexibility and fallibility within the human cognitive system. Furthermore, because the measures are also signs of the activity of the brain, they can be used to specify those brain structures involved in particular kinds of cognitive functions. The research will explore a model of human error detection/compensation that proposes that a comparison process detects when current actions deviate from correct actions. Mismatches computed by this comparison process may result in immediate attempts to inhibit or correct the error and strategic adjustments to prevent future errors. One common source of errors is an incompatibility between stimuli and responses. The research will also investigate models that attribute these kinds of errors to particular failures of the processing system. Since the cognitive deficits associated with a number of different psychopathologies and neuropsychological deficits can be characterized -in terms of cognitive inflexibility and extreme fallibility, the research will contribute to an understanding of the systems responsible for these deficits.